Infrared detector-based surveillance sensors ideally exhibit an extremely wide angular coverage, an ultra-compact size and high sensitivity and high image quality over the entire field-of-view (FOV). Typical applications include, but are not limited to, missile warning systems (MWS) and infrared search and track (IRST) systems. Another application of interest includes day/night area surveillance cameras that use optically-based wide-angle coverage, as opposed to mechanical scanning, for compactness and ruggedness.
Conventional wide-angle lens systems are typically used with dewar and coldshield technology. Due to the often-required small size of the IR sensor device itself, the use of re-imaging optics is generally precluded for many applications, resulting in the presence of only a single aperture stop or pupil for the entire optical system. In practice, the aperture stop is a “coldstop” located within the dewar to maintain infrared sensitivity. However, the resultant optical asymmetry complicates the lens design, thereby limiting image quality and image illumination. Further, if high image quality is maintained then a small physical size is difficult to achieve. In addition, the need for aberration corrections results in more stringent manufacturing tolerances, which increases cost.
One example of a conventional wide-angle IR sensor 1 is shown in FIG. 1. The sensor 1 includes an IR detector 2 located at an image plane 2A. The IR detector 2 is located within a coldshield 3 having an opening that defines a coldstop 3A. A dewar window 4 of a dewar housing 4A, also referred to herein simply as the dewar 4A, separates the coldstop 3A from uncooled optical components 5 that include a multi-element (four elements in this case) lens comprised of lens elements 6, 7, 8 and 9. A protective sensor dome 10 or window 11 defines the entrance to the IR sensor 1. Representative dimensions (inches) for a f:2.0 lens with 150° diagonal field-of-view coverage are: x=1.9, y=1.9 and z=2.6. The IR detector 2 can be said to be a “staring” type of detector, as the IR arriving from the scene is not scanned across the radiation-responsive surface of the detector 2.
In this embodiment the coldstop 3A is located at the true coldshield 3, which lies within the dewar 3. Designed for compactness, the distance from the focal plane 3A to the dome 10 exterior surface is only 1.9 inches. The window 11 may be employed if the sensor depth were increased, and if the sensor opening were increased to 2.6 inches (clear aperture diagonal) to accommodate the outward spread of the imaging rays with distance from the coldstop 3A.
This conventional design can be said to be optically asymmetric. This can be seen if one were to consider, by analogy, the aperture stop (coldstop 3A) as the fulcrum or pivot point of a beam having at one end the image plane 2A, and at the other end the front surface of the sensor dome 10. As can be appreciated, if the optical system where symmetric then the aperture stop would be located between lens elements 8 and 7, i.e., mid-way between the image plane 2A and the front surface of the sensor dome 10.
A second prior art sensor 1′ is shown in FIG. 2. The lens achieves a similar small size, and also uses a single stop 3A in the coldshield 3. Although designed for use with a flat window 11, a low-power dome 10 could be used instead (as in the FIG. 1 sensor), to reduce the aperture size of the sensor housing. Representative dimensions (inches) for an approximate 120° field-of-view are: x=1.7 and y=2.4. This lens produces a highly distorted image mapping that requires extensive electronic compensation. Otherwise, the performance and size are similar to that of the FIG. 1 lens.
A third prior art design is shown in FIG. 3A, and reflects the sensor shown in the now commonly-assigned U.S. Pat. No.: 4,820,923, “Uncooled Reflective Shield for Cryogenically-Cooled Radiation Detectors”, by William H. Weilman. A virtual coldshield, or “warmshield”, is used to avoid the large size of the coldshield 3 of the conventional design (FIG. 3B). One problem solved by the invention disclosed in U.S. Pat. No.: 4,820,923 was the presence of the large thermal mass and cantilevered weight of the conventional coldshield 3. The problem is solved by the use of the multiple toroidal reflectors, enabling the coldshield mass and length to be reduced. One significant advantage of the warm shield design of FIG. 3A is that the cryogenically cooled cold shield 3 can be made smaller, and can require less cooling, than the conventional cold shield 3 design of FIG. 3B.
It is noted that in FIG. 4 of U.S. Pat. No. 4,820,923 an optical element 30 is shown disposed between toroidal segments 26b and 26c (see column 6, lines 31–57). However, a consideration is not made for the advantages of lens symmetry that are important in a wide-angle optical system. FIG. 3A of this patent application shows that the geometry of this telephoto optical system is quite different than that of wide-angle optics.
Problems that arise when designing a wide FOV IR imaging system, that is intended to have a compact size, include a non-uniformity of sensitivity across the wide field-of-view, increased ghost image generation and limitations on spectral bandwidth.
A need thus exists to provide a wide FOV IR imaging system with a compact shape and improved optical characteristics that optical symmetry can help achieve. Prior to this invention, this need was not adequately addressed.